Department of Invertebrate Zoology News - No Bones
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18 April 2016

Spotlight on Antipatharians (Black Corals)

As the common name implies, black corals produce a skeletal axis that in some species can be jet black in color with a naturally high luster (Fig. 1A). Due to these attributes, black coral has for many centuries been used to make jewelry (Fig. 1C). It is not, however, an ideal substance for jewelry because, unlike precious red and pink coral, it is not stony in nature (i.e., does not contain calcium carbonate); instead, it is entirely organic and consists of concentric layers of protein and chitin (Fig. 1B). Consequently, over time, jewelry made from black coral is subject to drying out and cracking. In living black corals, however, the protein/chitin nature of the skeleton imparts a degree of flexibility to the thinner branches while increasing the rigidity of the thicker branches and stem which thereby provides greater support for the coral as it grows larger.

Black corals grow in many different, often tree-like patterns (Fig. 2), some of which look very similar to those of gorgonian corals, the familiar sea fans, sea feathers and sea whips found in shallow waters in subtropical and tropical regions. Colonies of some species can grow to a height of 3 m or more, whereas others may be no more than 10 cm tall. Sometimes the growth pattern of a colony can be highly uniform and highly symmetrical, which is often the case in deep-sea species (Fig. 2, lowermost row).

Figure 2. Colonies of black coral; those in the lowest row are representative of deep-sea species.

The skeleton of black corals is laid down by the overlying living tissue which in large colonies is composed of tens of thousands of tiny polyps each no more than a few millimeters in size (Fig. 3A–B). In most cases it is only with the aid of a hand lens that the individual polyps can be seen (Fig. 3C–D).

Antipatharian polyps are rather unique among anthozoan cnidarians (with the exception of a few sea anemones) in that they possess only six tentacles (Fig. 4A–E). Most other corals, sea anemones, ceriantharians (tube anemones), and zoanthids (colonial anemones) have more than six tentacles (Fig. 4F–I). The polyps of gorgonians (and other octocorals) have eight tentacles, each of which is pinnately branched like a feather (Fig. 4J). Polyps of black corals are also different from most other anthozoan groups in that they are not cylindrical in shape but instead are compressed (Fig. 4A), star-shaped (Fig. 4B) or stretched out in the direction of the branch (Fig. 4C). In addition, unlike other cnidarian polyps that usually have a flattened oral disc with the tentacles around the perimeter, black coral polyps do not have an oral disk and the tentacles can arise at different levels on the sides of the polyp.

In living colonies of black corals, the tentacles of the polyps, when fully expanded, can be five or more times the length they are when contracted (Figs 4D and E), and they may be so crowded together that they obscure the individual polyps (Fig. 4D). The surface of the tentacles may appear spotted due to the presence of distinct groupings of stinging cells (nematocysts) (Fig. 4A).

Depending on the species, the color of antipatharian polyps and the surrounding tissue (called coenenchyme) can vary from translucent or whitish to green, yellow, tan, orange, pink, purple or red (Figs. 5–6). Thus, living colonies of black corals do not appear black; it is only after the tissue is removed that the color of the skeleton is visible, and in many species the skeleton is not really black but various shades of brown, with the very thin branches often having a translucent reddish-brown appearance.

Another common name for black corals is “thorn corals” because the surface of the skeleton has microscopic spines – although in some species the spines are weakly developed and even absent on the thicker branches as shown in the specimen in Figure 1A. The spines are barely visible to the naked eye – usually appearing only as tiny spots on the surface of the coral. They become more noticeable when viewed with a hand lens (Fig. 7).

Figure 7. Magnified view of the branches of a black coral showing the skeletal spines.

The spines are rarely more than 0.5 mm in height, and range in shape from being compressed and triangular (when viewed from the side), to conical, thorn-like, blade-like, needle-like, and even blister-like (Fig. 8). In some species they become forked, or multiply lobed at the tip, and even antler-shaped in appearance (Fig. 9A–C)). The spines may be smooth or covered with minute surface features such as fine striations, papillae, or tubercles; these fine details may only be observable under the high magnification of a scanning electron microscope (Fig. 9D–F).

Figure 8. Various types of skeletal spines of black corals.

Antipatharians are exclusively marine and are found in all oceans from the Arctic to the Antarctic. The estimated number of known species is about 280. Some species of antipatharians can be found living at depths of only a few meters and others as deep as 8600 m, but most occur at depths ranging from about 20 to 1000 m. With a few exceptions, the colonies secrete a basal holdfast that allows them to permanently attach to a hard substrate.

Our current knowledge, based mainly on shallow-water species, suggests that all black coral species are dioecious, meaning that the polyps in a colony are either all male or all female. The eggs and sperm are released into the water column during mass spawning events similar to those observed in reef corals. The fertilized eggs develop into small ciliated balls of cells called planulae. The planulae of shallow-water species tend to settle on the bottom in locations where light intensity is minimal such as under reef overhangs, in crevices, and in underwater caves, or in areas with high levels of suspended particulate matter. For most species for which information is known the growth of a colony occurs at a rate of one or two centimeters a year. Recently one colony of black coral was estimated to have an age of over 4000 years, making that species one of the oldest living marine organisms.

The classification of the black corals has for many years been complicated by the description of numerous species from poorly preserved and/or incomplete specimens and by the lack of a clearly defined taxonomic hierarchy at the genus and family level. For a long time all species were placed in a single family, the Antipathidae, with many species of very diverse morphologies placed in the single genus Antipathes. For the past 15 years I have been attempting to establish a classification system based on morphological characters that reflect natural relationships within the order. As a result today seven families of antipatharians are recognized: Antipathidae, Schizopathidae, Cladopathidae, Leiopathidae, Myriopathidae, Aphanipathidae, and Stylopathidae. With some degree of overlap, these families tend to be separated by bathymetric distribution, with two families found in relatively shallow depths, two in intermediate depths, and three most often in the deep-sea. Families are morphologically differentiated on the basis of several taxonomic characters including the number of mesenteries (internal partitions) in the polyps; size and shape of the polyps; size and appearance of the tentacles of the polyps, and the general morphology of the spines. Within the families, genera are differentiated primarily by the growth form of the corallum. Species may be distinguished by one or more of the following features: a) relative length, thickness, density, and arrangement of the branches; b) arrangement of the branches and branchlets; c) size of the polyps; and d) size and morphology of the spines. In recent years the morphologically based classification has been put to the test with new genetic data. The advent of DNA studies has led to a confirmation of many parts of the new classification based on morphology; however, it has also produced several interesting surprises. In one case, three morphologically distinct genera could not be separated by their DNA; in another, a genus long thought to belong to one family based on the internal anatomy of the polyp was found to be associated with another in which it had only superficial morphological similarities; and in a third case, two families recognized on the basis of morphology were shown to be very similar genetically, suggesting that these families may eventually be relegated to subfamily status.

The primary challenge in working with black corals is that these colonial organisms have relatively few morphological characters, and all of these are highly variable within species, across species, and even across generic lines. As a result, the delineation of species and generic boundaries becomes more difficult as more and more specimens are examined. Although this creates a challenge for the taxonomist, it leads one to suspect that there may be biological mechanisms at work, such as a relatively high degree of natural hybridization and genetic plasticity that has evolved to insure not only the survival of individual species, but the group as a whole.

One final note. The scientific name for black coral is Antipatharia, derived from the genus name Antipathes, Greek for “against disease”, a reference to the common belief held by peoples in the Far East and in the Indo-Pacific, that corals of this type had mystical curative powers. Amulets of black coral were worn to ward off evil spirits, and there are even stories that black coral was ground into a powder and consumed as a medicine. In reality, because black coral is proteinaceous, it has the potential for just the opposite effect in that it might, in some individuals, produce a severe allergic reaction. There are, however, no reports of black coral jewelry causing a skin rash. Although black coral jewelry is available for purchase in Hawaii and in other localities in the Pacific and even in the Caribbean, it should be noted that all black coral is protected under CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora). Consequently, in most countries the collection of black coral is strictly controlled, and import and export of this material across international borders is strictly prohibited without governmental approval.